Evaluation on ecological environment system is to describe and assess the quality of ecological environment, and accurately reflect the ecological environmental quality and pollution situation. In this paper, after reviewing the domestic and foreign relative researches, from the theory perspective of the dissipative structure of complex system, we analyzed the city ecological system dissipative structure characteristic, and discussed the process forming dissipative structure, then we constructed the city ecosystem entropy parameter table and entropy evaluated model. The total entropy dynamic, scientifically reflects the city ecology system level and developing trend, so in fact it provides the theory basis and method to improve the overall quality of ecological environment, finally we took Dangyang as a case study, and got the same conclusion with the actual situation.